1. Field of the Invention
The present invention relates to a camera and, more particularly, it relates to a camera having a light measuring device.
2. Description of the Related Art
Heretofore, a camera in which various data including open diaphragm value, focal length, etc. peculiar to the camera are stored in memory means arranged in an exchangeable lens and these data are read by means of reading means provided in a camera body, whereby an exposure control value is decided on the basis of these data and light value obtained by a light measuring device provided in the camera has been already known in the art.
In order to decide the exposure control value, the camera has a light measuring device of so-called TTL type (through-the-lens type). In this type of light measuring device, it has been recently found that owing to difference between the respective cameras an error sometimes occurs in an output of such light measuring device relatively to a brightness on an image surface produced by the photographic lens of each of the cameras, for example due to difference of exit pupil positions of the respective photographic lenses even if these photographic lenses have same F-number. Such phenomenon will be described, with reference to FIG. 9.
FIG. 9 is a diagrammatic view showing essential parts of a camera, for explaining the phenomenon in which an error in output of the light measuring device can occur owing to difference in exit pupil position of the respective photographic lenses. In FIG. 9, 1 is a lens, 5 is a light receiving element, 20 is a total reflection mirror, 40 is a beam divider for light measurement and 60 is an image surface. The total reflection mirror 20 is arranged to direct an optical path toward the beam divider 40 or toward the image surface 60. The beam divider 40 consists of prisms 40a and 40b which are adhered together.
In the above-described construction, if the lens has an exit pupil located at substantially infinite distance a principal beam E1 at outside of the optical axis of the lens is incident upon the baas divider 40 in perpendicular direction thereto, while if the lens has an exit pupil relatively near to the image surface a principal beam E2 a outside of the optical axis of the lens is incident upon said divider at some angle thereto.
Accordingly, the beam E1 and the beam E2 which have been deflected at a semi-transparent portion 40c formed at the adhered surfaces of the prisms are emitted from said divider 40 at different angles to each other, so that the quantities of light received by the light receiving element 5 disposed at the side of said divider are slightly different from each other. Therefore, even if the brightness at the image surface 60, to which the beam is focused, is same, a slight difference in output of the light receiving element 5 is produced, depending upon the position of the exit pupil of the lens.
Such a phenomenon, which has been known in the art, presents an important problem particularly in a camera which makes use of a solid-state image sensor having a narrower latitude than that of a silver salt film.